Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Cardiovascular System
Blood Flow through Vessels

The Heart is the primary driving force behind the movement of Blood within the vascular bed. It Functions as both a suction (drawing blood from the Veins) and a pressure (pumping blood into the Arteries) pump.

Blood flows continuously through the vessels, closely synchronized with the PHASES OF CARDIAC activity. During ventricular systole, blood is ejected under high pressure, causing a rhythmic Displacement of the arterial walls known as the pulse. The pulse provides valuable insight into cardiac function, the state of The Cardiovascular system, and the overall condition of the body. Consequently, examining the pulse is an indispensable part of evaluating any patient or casualty. The primary focus during this assessment is on pulse rate, volume, and rhythmicity.

As a rule, the pulse rate equals the heart rate. In a healthy resting individual, it typically ranges from 60 to 80 beats per minute. Physical exertion, muscular work, prolonged walking, running, and elevated ambient temperatures all lead to an increased pulse rate. Tachycardia serves as a hallmark of febrile illnesses, wherein each 1 °C rise in body Temperature typically increases the pulse rate by an average of 8 to 10 beats per minute.

Blood pressure is one of the most critical indicators of cardiovascular function. A distinction is made between systolic and diastolic blood pressure. Systolic pressure depends primarily on Cardiac Output and the resistance of the arterial walls to blood flow. It is measured during systole, when the heart ejects a fresh volume of blood into the aorta and onward into the arteries. Diastolic pressure is determined by arteriolar resistance to blood flow. It is measured during diastole, as blood redistributes from the large arteries into smaller vessels. Systolic pressure exceeds diastolic pressure. The difference between systolic and diastolic pressure is known as pulse pressure.

Blood pressure can be measured using Direct and Indirect (non-invasive) Methods. The direct method involves inserting a needle connected to a manometer directly into the vascular lumen. Indirect methods are widely used clinically and represent the standard for examining any patient. Typically, the auscultatory cuff method introduced by Nikolai Korotkoff, a researcher at the Military Medical Academy, in 1905 is employed. To perform this measurement, a cuff is placed on the upper arm, and a stethoscope is positioned over the antecubital fossa. Air is pumped into the cuff to 160–180 mmHg or higher (as needed) and then slowly released. The appearance of auscultatory pulse sounds corresponds to the systolic blood pressure, while the disappearance of pulsations marks the diastolic blood pressure. It is recommended to repeat the measurement 2 to 3 times.

Normal systolic blood pressure in the brachial artery ranges from 120 to 130 mmHg, while normal diastolic pressure ranges from 70 to 80 mmHg. Measurement results are recorded by stating the systolic value, followed by the conjunction "over", and then the diastolic value. For example, a patient's blood pressure is recorded as 120 over 70 mmHg.

Pressure levels decrease as vessels extend further from the heart. The maximum pressure is observed in the aorta and major arteries; mean pressure in arterioles is 40–60 mmHg, and in capillaries, it drops to 15–20 mmHg. The lowest figures are characteristic of veins, ranging from 10 down to 1–3 mmHg (as they approach the heart). Thus, blood flows down a pressure gradient, moving from areas of higher pressure to lower pressure. The minimum blood velocity occurs in capillaries, which facilitates Metabolic exchange between Tissues and blood. Blood flow velocity is lower in veins than in arteries. The Venous system is estimated to hold 75–80% of the total blood volume at any given time, meaning these vessels serve a reservoir function. Changes in blood vessel diameter alter blood flow velocity and impact intravascular pressure.

The regulation of arterial blood flow is governed by The Nervous system and various humoral factors. The vasomotor center is located in the Medulla Oblongata and comprises pressor and depressor regions. Activation of the pressor region causes constriction of small arteries and enhances cardiac output; this effect is mediated by the sympathetic nervous system. The depressor region reduces Cardiac Activity via the Parasympathetic division of the Autonomic Nervous System. The parasympathetic nervous system has a significantly smaller influence on vessel caliber than the sympathetic system.

Vasopressin, adrenaline, noradrenaline, serotonin, and angiotensin act as vasoconstrictors and also increase heart rate. Prostaglandins, histamine, bradykinin, and acetylcholine produce the opposite effect.